The present invention relates to a spark plug which includes a partially cylindrical insulator element and a housing which surrounds the insulator element. The present invention also relates to a corresponding method of production. The insulator element typically includes a ceramic material. In contrast, the housing is made of metal.
Various methods are known for connecting the insulator element and the housing. Basically, these can be divided into hot assembly and cold assembly. In hot assembly, the insulator is inserted into the housing. The insulator is then pretensioned in the axial direction by reshaping an inwardly curved flange on the housing. The final pretension in the axial direction is achieved through a shrink fit process. During the shrink fit process, a shrinkage recess which surrounds the housing circumferentially is inductively heated to approximately 1050xc2x0 C. by a current pulse. As the shrinkage recess cools, the material in the region of the shrinkage recess shrinks. The housing is thus essentially secured on a projection of the insulator element by axial forces. At the same time, the housing is axially friction-locked between two shoulders of the insulator.
In cold assembly, a talcum powder packet is inserted between the flange, which is not yet curved, and the insulator element. Subsequently, the talcum powder packet is compressed by the reshaping process of the flange. In cold assembly as well, the insulator element must have a projection on which the inwardly curved flange is secured.
The known spark plugs do have connections which have high mechanical strength and are gas-tight, but they require a comparatively costly reshaping process.
An object of the present invention is the provision of a spark plug having a simple construction and a corresponding method of production, with the spark plug particularly being more compact than spark plugs, produced with typical methods of production, having similar or identical operating characteristics, e.g., with regard to thermal conductivity and with regard to electrical characteristics.
The present invention is based on the consideration that reshaping is only possible if the housing has a significantly larger diameter than the insulator element at the reshaping position. In addition, a peripheral projection of the insulator element in the region of the reshaping position must secure the housing.
In the spark plug according to the present invention, the insulator element and the housing are connected to one another by at least one material bond and/or one friction-lock connection aligned in the radial direction. The material bond is, e.g., a metallic soldered or welded connection and the radial friction-lock is a shrink fit connection.
This connection forms at least a significant portion of the cohesion of the housing and the insulator element. If the material bond and/or the friction-lock connection aligned in the radial direction absorb a part, e.g., approximately half, of the forces which act between housing and insulator element, reshaping can be reduced or even avoided completely, because the cohesion of insulator element and housing is achieved in another way. In addition, the peripheral projection on the insulator element can be designed smaller or even be dispensed with completely. If the other properties are unchanged, the spark plugs according to the present invention are more compact than comparable typical spark plugs, because the diameter of the housing selected can be smaller. Spark plugs according to the present invention have smaller internal thread diameters and smaller screw-in devices than known spark plugs having the same thermal value. For example, the outer diameter of the internal thread can be reduced from M14 to M12. Spark plugs produced until now with M8 threads can now be produced with M6 threads.
In a refinement of the spark plug according to the present invention, the diameter of the insulator core remains approximately the same or increases as the distance to the free end of the base part of the insulator (referred to in short in the following as base part) increases in the entire region surrounded by the housing. For example, the insulator core tapers in a stepped shape toward the free end of the base part. In other words, the insulator core does not have a projection in the region of the housing used to secure the housing and is therefore more compact than comparable known insulator elements.
In a subsequent refinement, the inner diameter of the housing in the region of the connection remains approximately the same or increases as the distance to the free end of the base part increases. In other words, the housing no longer has an edge which is curved inward. This allows the use of a housing with a smaller diameter, because reshaping of the edge is no longer necessary.
In a subsequent refinement, the diameter of the insulator element at the end further from the base part in the region adjoining the region surrounded by the housing is approximately equal to the largest diameter of the insulator core in the surrounded region. The insulator element is preferably cylindrical both inside a section of the housing and outside the housing, i.e., it has a uniform diameter. The fewer the projections and constrictions that are located on the insulator element, the more crack resistant it is.
In a subsequent refinement, the housing has at least one tubular section in which the diameter of the insulator core is only slightly smaller than the inner diameter of the housing lying at the same distance to the free end of the base part. The connection lies along the circumference of the insulator element in the gap between insulator element and housing. In this refinement, the connection has a double function, because it is used both for connecting insulator element and housing and for sealing the combustion chamber in which the spark plug is to be inserted.
The tubular section lies close to the base part and/or further from the base part. If the section is close to the base part, it is subjected to greater mechanical load and higher temperatures. On the other hand, the insulator element is thin near the base part, so that the circumference is smaller than further away from the base part. If the connection also seals the combustion chamber gas-tight, the combustion chamber is enlarged only insignificantly if the connection is near the free end of the base part. If the connection is at a greater distance from the free end of the base part, for example at the end of the housing further from the base part, the mechanical loads and the temperature effect are less. The connection will not be loaded as strongly during operation of the spark plug. If the connection is in multiple zones, the disadvantages of one position can be avoided by the advantages of the other position.
In embodiments, the connection is a soldered connection, e.g., a hard soldered connection, an active soldered connection, a welded connection, and/or an adhesive connection. For the welded connection, the known welding methods are used, e.g., friction welding or gas fusion welding. Reactive adhesives, whose components react during curing, are, for example, used as the adhesive for the adhesive connection. However, hard-setting adhesive materials whose components do not react during curing are, for example, also used.
In an alternative refinement, the housing contains at least one tubular section in which the diameter of the insulator element is slightly larger than the inner diameter of the housing, when the insulator element is not in place, lying at the same distance to the free end of the base part. Therefore, this is a compression connection, for example a longitudinal compression connection or a transverse compression connection. During the production of the transverse compression connection, for example, the housing is heated. Subsequently, the insulator core is inserted into the expanded housing. As the housing cools, it shrinks and tightly surrounds the insulator element.
In a refinement of the spark plug according to the present invention, insulator element and housing are connected with one another using an interlayer which was produced before housing and insulator were connected. The interlayer is produced from a material which is capable of being connected well on one side with the ceramic and on the other side with the metal of the housing. The interlayer can, for example, be formed by a thin sheet steel sleeve. However, interlayers made of other materials, e.g., plastic or glass melt, are also used. The interlayer is applied or attached to the insulator element. Thus, interlayers can be deposited directly on the insulator element. The interlayer is attached to the housing using a material bond and/or a friction-lock connection.
If, in an embodiment, the interlayer also extends into regions which lie outside the connection region, the interlayer can be attached better to the insulator, because the connection surface between the insulator and the interlayer is larger.
In a refinement, there is a gap between the housing and the interlayer in the region of the section lying closer to the base part. In the region of a section lying further away from the base part than this section, the interlayer is connected with the housing. In the section lying further away, the interlayer can also be connected with the insulator. However, in an alternative, there is a gap between interlayer and insulator in the section lying further away. In this refinement, a small peripheral ring of the interlayer is exposed in the gap between the insulator and housing. The ring-shaped section forms a kind of membrane which absorbs mechanical loads.
In refinements of the spark plug, the insulator element includes ceramic. The surface of the ceramic is treated in the region of the connection in such a way that the load capacity of the connection is enhanced. Roughening of the surface and/or applying a metallic topcoat are suitable methods.